With the growing demands for electric vehicles or hybrid vehicles in view of global environmental problems, rechargeable batteries used as power sources for such vehicles are desired to be smaller and lighter as well as to have higher capacity and power output, and battery packs consisting of a plurality of series-connected battery cells have been used for this application.
One example of the structure of a lithium ion battery that has conventionally been used is described below with reference to FIG. 6. Reference numeral 21 denotes an electrode assembly that is formed by winding together a positive electrode plate 22 consisting of a positive current collector 22b and positive electrode material 22a coated thereon, a negative electrode plate 23 consisting of a negative current collector 23b and negative electrode material 23a coated thereon, and a separator 24 interposed therebetween. Reference numerals 25, 26 respectively denote a positive and a negative current collector plate welded to end faces of the electrode assembly 21. A positive electrode tab 25a is welded to the positive current collector plate 25.
The electrode assembly 21 is accommodated in an outer case 27 with electrolyte, and the negative current collector plate 26 is connected to an inner bottom surface of the case 27 by resistance welding so that the outer case 27 serves as the negative terminal of the battery. Reference numeral 28 denotes a lid having a hole 28a in the center, which is joined to the case by caulking a caulking portion 28b at the outer edge after an O-ring 29, safety vent 30, spacer 31, and cap 32 are inserted therein. The safety vent 30 consists of a thin sheet of aluminum foil and breaks at a hole 31a in the spacer 31 upon an increasing of battery internal pressure above a predetermined level so as to release gas to the outside. The positive electrode tab 25a is welded to this lid 28, so that current from the electrode assembly 21 flows through the caulking portion 28b of the lid 28 to the cap 32, which serves as the positive terminal of the battery. Reference numeral 27a represents a groove formed in the outer case 27 by plastic deformation for determining the position of the lid 28. Reference numeral 33 denotes a gasket interposed between the outer case 27 and lid 28 for providing insulation therebetween; it also provides a seal when the open end edge 27b of the case 27 is caulked such as to sandwich the lid 28.
A plurality of batteries having the above structure are connected in series using suitable connecting members to form a battery pack as shown, for example, in Japanese Patent Laid-Open Publication No. Hei. 10-106533. The structure of the connecting part of this battery pack will be described with reference to FIG. 7. The connecting member 35 is a stepped cup-shaped pressed part including a bottomed small diameter cylindrical portion 36, step 37, and large diameter cylindrical portion 38. The bottom is formed with a hole 39 through which the connection boss on the cap 32 passes, and is in contact with the cap 32 and welded thereto at a plurality of projections or welds 40 by resistance welding. The bottom of the outer case 27 fits into the large diameter cylindrical portion 38 and rests on the step 37, and welded to the large diameter cylindrical portion 38 at a plurality of projections or welds 41 by resistance welding.
With the above structure of the conventional battery and battery pack, however, batteries are connected using the connecting members 35 separately of the battery's lid 28. The assembling process thus includes assembly of the battery pack, mounting of the connecting members 35, and connection of other batteries, i.e., it takes numbers of components and assembling steps and the costs high, resulting in a poor mass productivity.
Another problem is that the electrode assemblies 21 of adjacent batteries are connected to each other via the positive current collector plate 25, the positive electrode tab 25a, the lid 28, the cap 32, the connecting member 35, the outer case 27, and the negative current collector plate 26, i.e., because the current path between the cells is long and includes many connection points, the resistance is accordingly large. The large resistance per cell is a large inhibitor to longer battery life and higher power output.
In view of the problems encountered in the prior art, it is an object of the present invention to provide a battery and a battery pack, which are efficiently mass produced with fewer numbers of assembling steps and components and lower cost, and with which a higher power output is achieved by reducing internal resistance per cell.